1. Field of the Invention:
The present invention relates to a method of an apparatus for controlling a speed ratio of a continuously variable transmission for use on a motor vehicle, and especially a continuously variable transmission having a drive pulley on an input shaft, a driven pulley on an output shaft, and a V-belt trained around the drive and driven pulleys, the effective diameters of the pulleys being controlled for continuously varying the speed ratio between the input and output shafts.
2. Description of the Relevant Art:
There have been proposed in recent years automatic transmissions of the type in which the effective diameters of drive and driven pulleys with a V-belt trained therearound are controlled to continuously control the speed ratio. The recent trend is that in such belt-and-pulley type continuously variable transmissions a throttle valve opening degree and an engine rotational speed or a drive pulley rotational speed are employed as parameters for controlling the speed ratio.
For example, Japanese Laid-Open Patent Publications No. 55-65755 laid open May 17, 1980 discloses a V-belt-and-pulley type continuously variable transmission in which a pressure of working oil supplied from a pump is regulated in relation to the rotational speed of a drive pulley and the speed ratio, and the regulated oil pressure is supplied to the hydraulic cylinder of the movable conical member of the drive pulley through a control valve that is controlled according to the drive pulley rotational speed and the throttle valve opening degree for thereby controlling the speed ratio.
Japanese Laid-Open Patent Publication No. 60-98254 laid open Jun. 1, 1985 discloses an electronically controlled continuously variable transmission having a solenoid-operated valve for selectively supplying a regulated oil pressure to the hydraulic cylinders of drive and driven pulleys, the solenoid-operated valve being controlled by a signal indicating a throttle valve opening degree and a signal indicating a drive pulley rotational speed (input shaft rotational speed). More specifically, the solenoid-operated valve is controlled based on a signal indicating a throttle valve opening degree and a signal indicating a drive pulley rotational speed with reference to a stored map.
In the disclosed control methods, the speed ratio of the continuously variable transmission is controlled such that the engine rotational speed is determined solely with respect to the throttle valve opening degree, i.e., an engine power output can be produced correspondingly to a throttle valve opening degree. These control methods are preferred when the engine always operates in a range in which fuel economy is good, and is also advantageous in that they utilize the engine rotational speed which is indispensable in order to control the clutch disposed between the engine and the continuously variable transmission.
There are instances however where the input shaft rotational speed of the transmission and hence the speed ratio suddenly changes due to a change in running conditions which is caused by an increased road gradient or a head wind, even if the driver keeps on depressing the accelerator pedal to a constant depth. In such instances, it is difficult to keep the vehicle speed at a constant level. Stated otherwise, with the aforesaid belt-and-pulley type continuously variable transmission, the driver may have an unusual feeling such that the vehicle speed does not smoothly respond to the accelerator pedal which is directly operated by the driver. This problem manifests itself particularly when the driver, who is accustomed to a conventional automatic transmission employing a torque converter, operates the belt-and-pulley type continuously variable transmission.
According to a torque-converter type automatic transmission comprising a torque converter and a control valve, as disclosed in U.S. Pat. No. 4,579,208 issued Apr. 1, 1986, a shift valve is operated dependent on a vehicle speed and a throttle valve opening degree for controlling the speed ratio at discrete steps. In the automatic transmission of this type, the throttle valve opening degree is adjusted as usual by the driver who operates the accelerator pedal in view of a vehicle speed that can be felt by the driver himself. The speed ratio and the engine rotational speed follow the throttle valve opening degree until a desired vehicle speed is finally obtained. Therefore, the driver does not have a strange feeling as to the response of the vehicle speed to the depression of the accelerator pedal.
It would be practically infeasible to realize the method of controlling the speed ratio of the above torque-converter type automatic transmission by the aforementioned belt-and-pulley type continuously variable transmissions, since many shift valves would be required when doing so.
The present invention has been made in an effort to solve the problems of the aforesaid belt-and-pulley type continuously variable transmissions.